Effects of alkyl chain length on properties of 1-alkyl-3-methylimidazolium fluorohydrogenate ionic liquid crystals

Article abstract of DOI:10.1002/chem.201001364

A series of 1-alkyl-3-methylimidazolium fluorohydrogenate salts (C _xMIm(FH)₂F, x=8, 10, 12, 14, 16, and 18) have been characterized by thermal analysis, polarized optical microscopy, IR spectroscopy, X-ray diffraction, and anisotropic ionic conductivity measurements. Liquid crystalline mesophases with a smectic A interdigitated bilayer structure are observed from C₁₀ to C₁₈, showing a fan-like or focal conic texture. The temperature range of the mesophase increases with the increase in the alkyl chain length (from 10.1°C for C₁₀MIm(FH)₂F to 123.1°C for C₁₈MIm(FH) ₂F). The distance between the two layers in the smectic structure gradually increases with increasing alkyl chain length and decreases with increasing temperature. Conductivity parallel to the smectic layers is around 10mS-cm^?1 regardless of the alkyl chain length, whereas that perpendicular to the smectic layers decreases with increasing alkyl chain length because of the thicker insulating sheet with the longer alkyl chain.

Full text of DOI:10.1002/chem.201001364

DOI: 10.1002/chem.201001364
Effects of Alkyl Chain Length on Properties of 1-Alkyl-3-methylimidazolium
Fluorohydrogenate Ionic Liquid Crystals
[
a]
Fei Xu, Kazuhiko Matsumoto,* and Rika Hagiwara
Abstract: A series of 1-alkyl-3-methyli-
midazolium fluorohydrogenate salts
focal conic texture. The temperature
range of the mesophase increases with
the increase in the alkyl chain length
(from 10.18C for C MIm(FH) F to
smectic structure gradually increases
with increasing alkyl chain length and
decreases with increasing temperature.
Conductivity parallel to the smectic
layers is around 10 mScm regardless
of the alkyl chain length, whereas that
perpendicular to the smectic layers de-
creases with increasing alkyl chain
length because of the thicker insulating
sheet with the longer alkyl chain.
(
C MIm(FH) F, x=8, 10, 12, 14, 16,
x 2
and 18) have been characterized by
thermal analysis, polarized optical mi-
croscopy, IR spectroscopy, X-ray dif-
fraction, and anisotropic ionic conduc-
tivity measurements. Liquid crystalline
mesophases with a smectic A interdigi-
tated bilayer structure are observed
1
0
2
ꢀ
1
123.18C for C MIm(FH) F). The dis-
1
8
2
tance between the two layers in the
Keywords: conducting materials ·
ionic liquids
mesophases
· liquid crystals ·
from C10 to C , showing a fan-like or
18
Introduction
served for these salts rapidly increases with increasing alkyl
chain length, although the alkyl chain length at which liquid
crystalline mesophases appear depends on the anionic spe-
cies.
Ionic liquids are widely studied as reaction media and elec-
trolytes because of their unique properties, such as wide
liquid temperature range, negligible vapor pressure, and
nonflammability. In particular, imidazolium-based ionic
liquids attract considerable attention since they give low
melting points with high chemical and electrochemical sta-
bility. Ionic liquid crystals (ILCs) can be considered as sub-
stances that possess the properties of both ionic liquids and
liquid crystals, being composed of ionic species and exhibit-
ing a liquid crystalline mesophase in a certain temperature
Ionic liquid crystals are interesting candidates to design
anisotropic ion-conductive materials since they have an ani-
sotropic structural organization composed of only ionic spe-
[1]
[2]
cies. Depending on the types of liquid crystalline meso-
[8–9]
[10–11]
phases, 1D
or 2D
ion conduction can be achieved by
use of fan-shaped or smectic ILCs, respectively, where the
high anisotropy in ionic conductivity can be obtained in
both the cases. However, the conductivity of these aniso-
tropic ion-conductive materials is not sufficient for practical
[
2]
range. Because of the widespread use of imidazolium-
based ionic liquids, imidazolium-based ILCs are frequently
0
1
ꢀ1
applications (10 –10 mScm ) so far. Although one type of
columnar liquid-crystalline assembly formed through nonco-
valent intermolecular interactions was reported to exhibit
[3]
studied. For example, 1-alkyl-3-methylimidazolium cations
with long alkyl chains were combined with various anions,
[
4]
[5]
ꢀ1
such as chloride (Cl), bromide (Br), tetrachlorometallate
ionic conductivities between 1 and 10 mScm at ambient
[4]
[6]
[9]
(
MCl , M=Co and Ni), hexafluorophosphate (PF ), tet-
temperature, further improvement in ionic conductivity at
4
6
[7]
rafluoroborate (BF ),
and trifluoromethanesulfonate
room temperature is still in great demand.
4
[5]
(
OSO CF ). The temperature range of the mesophase ob-
Since the first report of 1-ethyl-3-methylimidazolium fluo-
2
3
[12]
rohydrogenate in 1999,
a series of fluorohydrogenate
[
a] F. Xu, Prof. K. Matsumoto, Prof. R. Hagiwara
Graduate School of Energy Science
Kyoto University, Yoshida, Sakyo-ku
Kyoto 606-8501 (Japan)
Fax : (+81)75-753-5906
E-mail: k.matsumoto@ky7.ecs.kyoto-u.ac.jp
ionic liquids with alkylimidazolium, alkylpyridinium, alkyl-
pyrrolidinium, and alkylpiperidinium cations has been pre-
[13–16]
pared and characterized.
Fluorohydrogenate ionic liq-
ꢀ
uids contain the (FH) F anions exhibiting high ionic con-
n
ꢀ
1
ductivities (100 mScm
for 1-ethyl-3-methylimidazolium
fluorohydrogenate, EMIm(FH) F). Such a high conduc-
[14]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001364.
2.3
tivity of fluorohydrogenate ionic liquids makes them attrac-
12970
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 12970 – 12976

FULL PAPER
tive materials as electrolytes in various electrochemical devi-
ces, such as electrochemical capacitors and fuel cells.
Herein, a series of 1-alkyl-3-methylimidazolium fluorohy-
with a bent molecular shape (C ) was confirmed by broad
2v
[17]
[18]
absorption bands at approximately 450, 1050, 1800, 2000,
ꢀ
1
and 2350 cm in the IR spectra of all the C MIm(FH) F
x
2
[21]
drogenate salts (C MIm(FH) F, x=8, 10, 12, 14, 16, and 18)
salts
(Figure S1,
Supporting
Information).
The
x
2
were characterized by several techniques. Incorporation of
C MIm(FH) F salts (x=8, 10, and 12) are viscous liquids at
x 2
room temperature and the viscosity appears to increase with
ꢀ
(
FH) F into the ionic liquid crystals can lead to high aniso-
n
tropic ionic conductivity, since fluorohydrogenate ionic liq-
uids exhibit high conductivity. Such materials are potential
an increase in x. The C MIm(FH) F (x=14, 16, and 18) salts
x
2
are wax-like solids at room temperature. Hygroscopicity and
solubility in water of these fluorohydrogenate salts decrease
as the alkyl chain becomes longer.
electrolytes in electrochemical systems in which the diffu-
ꢀ
sion of (FH) F is involved with electrode reactions includ-
n
[18]
ing fluorohydrognate fuel cells and electrochemical fluori-
[19]
nation. The imidazolium cations with a normal alkyl chain
were chosen because the simple structure with high chemi-
cal and electrochemical stability is preferable to see the ef-
Thermal properties: Thermogravimetric (TG) analysis re-
vealed that C MIm(FH) F slowly loses HF at elevated tem-
x
2
peratures and decomposes around 2308C regardless of the
alkyl chain length (Figure S2, Supporting Information). This
behavior is similar to the other fluorohydrogenate salts with
[1]
fects of alkyl chain length on liquid crystalline behavior.
Physicochemical properties and structures of the obtained
fluorohydrogenate salts were studied by IR spectroscopy,
differential scanning calorimetry (DSC), polarized optical
microscopy (POM), and X-ray diffraction (XRD). Aniso-
tropic conductivity parallel and perpendicular to smectic
layers was measured for the liquid crystalline mesophase.
[13]
short alkyl chains. The DSC curves of C MIm(FH) F are
x
2
shown in Figure 1 and the DSC data obtained (transition
temperatures, DH, and DS) are summarized in Table 1. Only
the heating process is shown for each sample, since the fluo-
rohydrogenate salts slowly liberate HF at temperatures
[20]
above 458C, leading to a decrease in the n value, and pre-
cise measurement on the cooling process was difficult. For
C MIm(FH) F (x=12, 14, 16, and 18), two endothermic
Results and Discussion
x
2
peaks are observed in the thermograms. The endothermic
Synthesis: The fluorohydrogenate salts in the present study
were prepared by the reaction of the corresponding chloride
[12–15,17]
and a large excess of anhydrous HF.
When the alkyl
chain is short, the vacuum-stable HF composition in fluoro-
+
hydrogenate ionic liquids is 2.3 (namely, Cat (FH) F
(
2
.3
+
Cat =cation)) at room temperature regardless of the cat-
ionic structure (imidazolium, pyridinium, pyrrolidinium, and
[12–15,17]
piperidinium),
although this is not the case when the
resulting salt is solid at room temperature. At this composi-
ꢀ
ꢀ
tion, the two anions (FH) F and (FH) F are mixed in the
2
3
ratio of 7 to 3 to satisfy the composition. The present study
reveals that extension of the alkyl chain seems to break this
rule and the resulting vacuum-stable HF composition n for
C MIm(FH) F (x=8, 10, 12, 14, 16, and 18) at room temper-
x
n
Figure 1. Differential scanning calorimetric curves (heating process) for
ature ranges from 2.0 to 2.3, which probably arises from the
stronger hydrophobic interactions between the chains by the
introduction of a long alkyl chain. To see the effects of the
alkyl chain length on the C MIm(FH) F salts, the n value
x 2
C MIm(FH) F (x=8, 10, 12, 14, 16, and 18).
Table 1. Summary of the DSC analysis for C MIm(FH) F (x=8, 10, 12,
14, 16, and 18).
x
2
x
n
[
a]
ꢀ1
ꢀ1 ꢀ1
was adjusted to 2.0 by either removing HF at elevated tem-
peratures or mixing two fluorohydrogenate salts with n
values less than and greater than 2.0 (see Scheme 1 for the
x
Transition
cryst–iso
cryst–SmA2
SmA –iso
cryst–SmA
SmA –iso
T [8C]
DH [kJmol
]
DS [Jmol K ]
8
10
ꢀ28.1
ꢀ0.1
12.4
18.0
49.0
64.3
+
ꢀ
[b]
[b]
[b]
2
10.0
20.2
–
–
structures of C MIm and (FH) F ). All of the resulting
x
2
12
14
16
18
2
2
2
19.9
0.11
20.1
0.31
21.7
0.43
27.7
0.43
66.1
0.33
65.1
0.81
66.9
1.01
81.4
0.93
salts do not have a detectable HF dissociation pressure up
2
61.5
29.0
109.2
45.5
152.6
62.2
ꢀ
to temperatures around 458C. The (FH) F anions can be
n
cryst–SmA
[20]
ꢀ
best identified by IR spectroscopy.
The (FH) F anion
SmA –iso
2
2
cryst–SmA
SmA
cryst-SmA
SmA –iso
2
-iso
2
2
188.8
[
a] The symbols cryst, iso, and SmA
smectic A liquid crystal. [b] This transition temperature was determined
by XRD and DH and DS could not be obtained because the DSC peak
overlapped the peak of the cryst–SmA transition.
2
denote crytstal, isotropic liquid, and
2
+
ꢀ
Scheme 1. Structures of a) C
x
MIm and b) (FH)
2
F .
2
Chem. Eur. J. 2010, 16, 12970 – 12976
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12971

K. Matsumoto et al.
peaks at the lower temperature (with a large DH) and the
higher temperature (with a small DH) correspond to the
melting point (from crystal to liquid crystal) and the clearing
temperature (from liquid crystal to isotropic liquid), respec-
tively. The relatively large DH (and thus DS) on melting in-
dicates a considerable structural change, namely, the break-
up of a three-dimensionally ordered crystal lattice. The
small DH (and thus DS) at the clearing temperature is
mainly caused by the breakup of van der Waals interactions
between the alkyl chains, which is reflected in the increase
of DH (and thus DS) at clearing temperatures with the in-
crease in x. Similar phenomena were also observed for
ILCs, 1-alkyl-3-methylimidazolium chloride, bromide, hexa-
arises from characteristics of the anion as is the case for
ꢀ
[14]
ionic liquids based on (FH) F .
n
Structural properties: Figure 3 shows a polarized optical mi-
croscopic (POM) texture of C MIm(FH) F, which is typical
1
2
2
of the C MIm(FH) F salts (x=12, 14, 16, and 18) and char-
x
2
acterized by spontaneous formation of smooth fan-like or
[5–7]
fluorophosphate, and tetrafluoroborate.
melting peaks on the heating process may be ascribed to the
The small pre-
[22]
change in conformation of the alkyl chains. Although de-
tection of the liquid crystal to isotropic liquid transition was
difficult for C MIm(FH) F and C MIm(FH) F by DSC, the
8
2
10
2
results of POM and XRD indicated C MIm(FH) F has a
1
0
2
liquid crystalline mesophase with the clearing temperature
of about 108C.
Figure 2 shows melting points and clearing temperatures
of C MIm(FH) F plotted as a function of x. The clearing
2
Figure 3. Polarized optical microscopic texture of C12MIm(FH) F at 508C.
x
2
temperature shows a larger dependence on the alkyl chain
focal conic textures. A similar texture was observed for
C MIm(FH) F, but was much dimmer than the other cases,
1
0
2
probably because of the weak alkyl–alkyl interaction for the
shorter alkyl chain. The broken fan-like texture which is in-
dicative of the smectic C mesophase was not observed for
C MIm(FH) F in the liquid crystalline mesophase on cool-
1
2
2
ing from the isotropic phase to the crystalline phase, sug-
gesting the absence of the smectic C mesophase in this
[23–24]
system (Figure S3, Supporting Information).
These re-
sults of POM suggest that the liquid crystalline mesophase
[2,4,6–7]
of C MIm(FH) F is assigned to smectic A mesophase.
x
2
Such an enantiotropic smectic A liquid crystalline meso-
phase was also observed for other 1-alkyl-3-methylimidazoli-
um ILCs in previous reports, since the cation dominates for-
Figure 2. Melting points (&) and clearing temperatures (*) observed in
the DSC thermogram (Table 1) of C
8).
x 2
MIm(FH) F (x=10, 12, 14, 16, and
[4–7]
mation of the smectic layer.
These observations stand in
1
contrast to ILCs based on pyrrolidinium, piperidinium, pi-
perazinium, and morpholinium, which show a variety of tex-
[
25]
length (from 10.0 to 188.88C) compared with the melting
point (from ꢀ0.1 to 62.28C), leading to the wider tempera-
ture range of liquid crystalline mesophase for the cation
with a longer alkyl chain (e.g., 10.18C for C MIm(FH) F
tures because these ILCs exhibit various mesophases.
Confirmation of liquid crystal–isotropic liquid transition by
POM was technically difficult for some cases (x=14, 16, and
18) because of the high levels of HF dissociation at their
clearing temperatures.
1
0
2
and 126.68C for C MIm(FH) F). This is mainly due to the
18
2
increase in van der Waals interactions, as observed for other
Figure 4 shows the XRD patterns of the crystalline phase
and liquid crystalline mesophase for C MIm(FH) F (x=10,
ꢀ
ꢀ
ꢀ
1
-alkyl-3-methylimidazolium ILCs with Cl , Br , BF , and
4
x
2
ꢀ
[4,6–7]
PF .
The temperature range of the liquid crystalline
12, 14, 16, and 18) in the low-angle region (28<2q<68) and
Table 2 lists the layer spacings (d) for them (see Figure S4
and S5 in the Supporting Information for the XRD patterns
of the crystalline phase and liquid crystalline mesophase, re-
spectively, in the high-angle region, 58<2q<308). Sharp
peaks are observed in the low-angle region for both the
crystalline phase and liquid crystalline mesophase, indicating
formation of layered structures. No peak was found in the
region lower than 2.08 in either case. The peaks of
6
ꢀ
ꢀ
ꢀ
mesophase follows the sequence Cl >Br >(FH) F >
BF4 >PF6 >OSO CF , which can be explained by the
2
ꢀ
ꢀ
ꢀ
2
3
ability of the anion to three-dimensionally form hydrogen
[5]
bonds
with
an
imidazolium-ring
proton.
The
C MIm(FH) F salts (x=14, 16, and 18) show a relatively
x
2
wide temperature range of liquid crystalline mesophase al-
though it is not as wide as the chloride and bromide salts.
ꢀ
The low melting point observed for ILCs based on (FH) F
2
12972
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Chem. Eur. J. 2010, 16, 12970 – 12976

Properties of Fluorohydrogenate Ionic Liquid Crystals
FULL PAPER
Figure 5. Layer spacings of C
x 2
MIm(FH) F (x=10, 12, 14, 16, and 18) in
the crystalline phase (
&
) and liquid crystalline mesophase (*). See
Figure 4 for the measurement temperature of each sample.
Figure 4. X-ray diffraction patterns for C
and 18) in a) crystalline phase (ꢀ608C for C10, ꢀ208C for C12 and C14,
08C for C16, and 508C for C18) and b) liquid crystalline mesophase
58C for C10, 308C for C12, 408C for C14, 508C for C16, and 658C for
x 2
MIm(FH) F (x=10, 12, 14, 16,
1
(
C18).
Table 2. Layer spacings (d) for C
x
MIm(FH)
2
F (x=10, 12, 14, 16, and 18).
d in liquid crystal [ꢁ]
x
d in crystal [ꢁ]
10
12
14
16
18
21.7 at ꢀ608C
26.6 at ꢀ208C
29.6 at ꢀ208C
32.0 at 108C
33.7 at 508C
28.6 at 58C
30.4 at 308C
34.2 at 408C
36.8 at 508C
39.0 at 658C
Figure 6. Schematic illustration of the interdigitated bilayer structure of
the smectic A liquid crystalline mesophase.
2
ꢀ
ꢀ
salts with various anions follows the sequence Cl >Br >
ꢀ
ꢀ
ꢀ
(
FH) F >BF >OSO CF . This is the same as the se-
2 4 2
3
C MIm(FH) F (x=14, 16, and 18) are sharper than those of
quence of temperature range for the ILC mesophase and
can be explained by the different anion–cation interactions
within the polar region including hydrogen bonding.
x
2
C MIm(FH) F (x=10 and 12) in the liquid crystalline meso-
x
2
phase, indicating the more highly ordered structure of the
cations with the longer alkyl chain. A small peak ascribed to
the supercooled ILC was also observed at the low 2q-angle
side of the main peak in the case of C MIm(FH) F (2q=
Figure 7 shows XRD patterns of C MIm(FH) F in the
1
2
2
crystalline phase and liquid crystalline mesophase at differ-
ent temperatures. Figure 8 shows the layer spacing against
temperature for C MIm(FH) F and Table 3 lists the layer
1
0
2
3.708). A similar phenomenon was also observed for
1
2
2
C MIm(FH) F when the cooling rate was fast. Absence of
spacing at each temperature. The shift of the main peak in
the crystalline phase is negligible, whereas that in the liquid
crystalline mesophase shifts to the high angle with an in-
12
2
additional peaks in the higher 2q-angle region (68<2q<
08) suggests the loss of positional ordering within the layer
3
plane (Figure S5, Supporting Information). To our knowl-
+
edge, this is the first case of the ILC having the C MIm
1
0
cation.
Figure 5 shows layer spacings (d) of C MIm(FH) F (x=
x
2
10, 12, 14, 16, and 18) plotted against x. The layer spacing
increases gradually against x in both the crystalline phase
and liquid crystalline mesophase. The layer spacing d is
found to satisfy l<d<2l, where l is the fully extended
length of the cation and is roughly estimated from crystallo-
[26]
graphic data. This result indicates an interdigitated bilayer
structure is formed in the smectic A liquid crystalline meso-
[5]
phase (smectic A2) as shown in Figure 6. It is likely that
the alkyl chain of the cation is tilted with respect to the
[5–6]
layer plane in the crystalline phase,
since the smaller
layer spacings are observed in the crystalline phase com-
pared with the liquid crystalline mesophase. By taking previ-
Figure 7. X-ray diffraction patterns in the low-angle region of a) crystal-
line phase and b) liquid crystalline mesophase for C MIm(FH) F at dif-
[5]
ous data into account,
the layer spacing for smectic A2
1
2
2
liquid crystalline mesophase of 1-alkyl-3-methylimidazolium
ferent temperatures.
Chem. Eur. J. 2010, 16, 12970 – 12976
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12973

K. Matsumoto et al.
applied (Figure S11, Supporting Information). Rotation of
the sample on the stage did not change the vision, indicating
[24]
formation of a homeotropic alignment.
Figure 9 shows the temperature dependence of s and s
k
?
for C MIm(FH) F (x=10, 12, 14, and 16) (See Table S2 in
x
2
the Supporting Information for the s and s values at each
k
?
Figure 8. Layer spacings of C12MIm(FH)
2
F in crystalline phase ( ) and
&
liquid crystalline mesophase (*) at different temperatures.
Table 3. Layer spacings (d) for C12MIm(FH)
2
F at different temperatures.
T [8C]
d in crystal [ꢁ]
d in liquid crystal [ꢁ]
ꢀ
ꢀ
ꢀ
ꢀ
40.0
30.0
20.0
10.0
26.6
26.6
26.6
26.6
26.7
26.6
–
–
–
–
–
–
–
–
Figure 9. Temperature dependence of anisotropic ionic conductivity for
MIm(FH) F (s
: x=10 (*), 12 (~), 14 (
), and 16 (^); s : x=10 (*),
2 (~), 14 (&), and 16 (^)).
C
1
x
2
k
&
?
0
.0
–
1
2
3
4
5
0.0
0.0
0.0
0.0
0.0
31.5
30.9
30.4
30.0
29.6
temperature). The C MIm(FH) F salt does not exhibit sig-
10
2
nificant anisotropy in ionic conductivity. This is probably be-
cause the decyl group is not long enough to form a highly
ordered mesophase, although the POM somewhat showed
formation of structural domains. For the C MIm(FH) F salts
[5]
crease in temperature. In the smectic C mesophase, the
tilted angle of the layer structure decreases with increasing
temperature, leading to an increase of the layer spacing
x
2
(x=12, 14, and 16), s is at least ten times higher than s ,
k
?
suggesting the high ion mobility in the ionic layer as shown
in Figure 6. The conductivity of ionic liquids based on 1-
alkyl-3-methylimidazolium cation usually decreases with in-
creasing alkyl chain length because of the increase in viscos-
(
Figure S6, Supporting Information), whereas in the smectic
A mesophase, the bilayer structure interdigitates more
deeply with increasing temperature due to the increase of
thermal mobility of the alkyl chains, leading to a decrease of
the layer spacing. The observation for C MIm(FH) F fits
[14]
ity,
whereas s for C MIm(FH) F (x=12, 14, and 16)
k x 2
shows a higher conductivity than C MIm(FH) F owing to
1
2
2
10
2
the latter case, which agrees with the results of POM.
formation of the well-ordered ion-conductive layers. The
slight decrease in s with increasing x from 12 to 16 may be
k
Anisotropic ionic conductivity: Conductivity parallel (s )
explained by reduction of the number of ion-conductive
layers per unit length if the mobility of the ionic species in
the layer is the same in the three salts. As was suggested for
k
and perpendicular (s ) to the smectic layer was measured
?
for C MIm(FH) F (x=10, 12, 14, and 16). The cell using
x
2
ꢀ
[11]
gold and ITO glass electrodes was used for this measure-
ment (See Figures S7–S10 in the Supporting Information
the IꢀI system in a smectic layer, the dominant charge
3
carrier in these ion-conductive layers is thought to be
[
8–11]
ꢀ
and references
drogenate salts at elevated temperatures reacted with the
glass substrate, s and s could be measured up to 45 and
). Since HF liberated from the fluorohy-
(FH) F , since the cation is much larger than the anion and
2
linked to the adjacent cation by van der Waals interactions.
Such a conduction mechanism implies application of the flu-
orohydrogenate ILCs as electrolytes in the electrochemical
systems in which the fluorohydrogenate anion is involved
k
?
658C, respectively. Measurement of the conductivity of
C MIm(FH) F was difficult due to its relatively high melt-
18
2
ing point (62.28C). In contrast to the other ILCs reported
with the electrode reactions. On the other hand, s shows a
?
previously, cooling down from the isotropic phase or shear-
much sharper decrease with increasing alkyl chain length
[8–10]
ing in the liquid crystalline mesophase
is not indispensa-
than s , leading to the increase in anisotropy of ionic con-
k
ble for the alignment of these fluorohydrogenate ILCs,
probably due to their simple structures and relatively lower
viscosity. Focal conic textures along with pseudo-isotropic
domains were observed when the sample was placed in the
conductivity measurement cell, whereas it turned into a to-
tally pseudo-isotropic state when a pressure on the glass was
ductivity from C12 to C . This observation indicates the in-
sulating alkyl chain layers are developed more by introduc-
16
ing the longer alkyl chain. The discontinuous gap of s at
?
the clearing temperature of C MIm(FH) F (61.58C) results
1
2
2
from the breakup of the layer structure. Temperature de-
pendence of s is similar to that of s . Previous work on
k
?
12974
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Chem. Eur. J. 2010, 16, 12970 – 12976

Properties of Fluorohydrogenate Ionic Liquid Crystals
FULL PAPER
the conductivity of some other ion-conductive liquid crystals
Gold (about 0.8 mm in thickness) was deposited on a borosilicate glass
substrate in a comb shape after treatment of indium tin oxide (ITO) dep-
osition (about 0.1 mm) in the same pattern to reinforce the contact on the
glass. The sample was placed in the comb-shaped region covered with a
piece of glass substrate to help the alignment. A pair of ITO (indium tin
[8–11,27]
also reported the same results.
Although the mecha-
nisms of ion transport perpendicular to and parallel to the
smectic layer are different to each other, these results sug-
gest the activation energies are not.
oxide) glass electrodes was used to measure s
?
(Figure S9 and S10, Sup-
[
8–10,27]
porting Information).
The sample was sandwiched with two ITO
glass electrodes and fixed with a Teflon spacer (50 mm in thickness). The
cell constants of these cells were calibrated with a KCl aqueous solution
Conclusion
ꢀ1
(
0.747 gkg ), EMImBF
4
(Kanto Kagaku) and BMImPF
6
(Kanto
Kagaku). These conductivity measurement cells were placed in an air-
tight cell to avoid the effects of water (see Figure S12 in the Supporting
Information for a schematic drawing of the airtight cell). The cell was
held at each temperature for more than one hour to obtain a steady re-
sistance and the measurement was repeated several times to confirm re-
producibility of the data. No etching of the glass surface was observed
after the mesurement.
The thermal, structural, and ion-conductive properties of 1-
alkyl-3-methylimidazolium
fluorohydrogenate
ILCs,
C MIm(FH) F (x=10, 12, 14, 16, and 18), were investigated.
x
2
The ILCs show a smectic A texture in the liquid crystalline
mesophase and the temperature range of the mesophase in-
creases with increasing alkyl chain length. The layer spacing
of the interdigitated bilayer structures increases with in-
creasing alkyl chain length both in the crystalline phase and
liquid crystalline mesophase. The large anisotropy was ob-
served in the ionic conductivity of C MIm(FH) F (x=12, 14,
x 2 x
Synthesis of C MIm(FH) F: The starting chloride, C MImCl, was weigh-
ed in a PFA reactor under a dry Ar atmosphere and a large excess of an-
hydrous HF was distilled on that at ꢀ1968C. The mixture reacted on
warming up to room temperature and the volatile gases were roughly
eliminated by evacuation using a rotary pump. Elimination of the volatile
gases and addition of fresh HF were repeated for effective elimination of
chloride in the form of hydrogen chloride from the salt. Fluorohydrogen-
x
2
and 16) because of formation of the smectic layer structure
and the degree of the anisotropy increases with the increase
in the alkyl chain length of the cation.
x n
ate salts C MIm(FH) F with the specific HF composition of 2.0 were pre-
pared by removing HF at elevated temperatures or mixing two fluorohy-
drogenate salts with different n values (less than and greater than 2.0).
The HF composition of the obtained salts was confirmed by elemental
analysis and titration using aqueous 0.1029m NaOH (see Table S1 in the
Supporting Information for the results of determination of the HF com-
position).
Experimental Section
General: Volatile materials were handled in a vacuum line constructed of
SUS316 stainless steel and PFA (tetrafluoroethylene-perfluoroalkylviny-
lether copolymer). Nonvolatile materials were handled under a dry Ar
Acknowledgements
atmosphere in a glove box. The starting chlorides, C
x
MImCl (x=8, 10,
1
2, 14, 16, and 18), were prepared by reactions of 1-methylimidazole (Al-
drich, 99%) and an equimolar quantity of the corresponding chloroal-
kanes (1-chlorooctane (Wako Chemicals, 95%), 1-chlorodecane (Wako
Chemicals, 95%), 1-chlorododecane (TCI-EP, 97%), 1-chlorotetradecane
This work was financially supported by a Grant-in-Aid for Scientific Re-
search from the Japan Society for the Promotion of Science, 20246140.
The authors gratefully acknowledge the advice on conductivity measure-
ment by Prof. Hiroyuki Ohno of Tokyo University of Agriculture and
Technology.
(
Aldrich, 98%), 1-chlorohexadecane (Wako Chemicals, 95%), 1-chlor-
ooctadecane (Wako Chemicals, 95%)) at temperatures ranging from 70
[
14]
to 1008C for several days as described in the literature. Purification of
the chlorides was performed by dissolving the salts in acetonitrile (dehy-
drated, Wako Chemicals, 99%) and then precipitating from the solution
by adding ethyl acetate (dehydrated, Wako Chemicals, 99.5%). Anhy-
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Chem. Eur. J. 2010, 16, 12970 – 12976